Both 1-methylnaphthalene and 2-methylnaphthalene are useful materials of commerce. 2-methylnaphthalene can be oxidized to vitamin K.sub.3 (menadione). 1-methylnaphthalene is useful as a fuel additive, as a plasticizer in polyvinyl chloride, as an intermediate for preparing rubber additives and as an intermediate for preparing fungicides.
The preparation of 1-methylnaphthalene and 2-methylnaphthalene by reacting methyl chloride with naphthalene using alumina catalyst is known [J. Am. Chem. Soc. 67, 1312-15 (1945)]. This article states that the optimum temperature was 400.degree. C.
The use of methanol, instead of methyl chloride, as an alkylating agent provides several advantages. Usually, it does not produce a product with as much polyalkylate as alkylation with methyl chloride does. Use of methyl chloride results in hydrogen chloride (HCl) as a coproduct. HCl is known to be corrosive and toxic. On the other hand, the only coproduct with the use of methanol as the alkylating agent is water, which is innocuous. Methanol is less toxic than methyl chloride, particularly as an atmospheric contaminant. Since methanol is a liquid, it is easier to handle and to contain than methyl chloride, which is a gas under normal conditions. Moreover, methanol is less expensive than methyl chloride on a mole basis.
I have discovered a process for making 1-methylnaphthalene and 2-methylnaphthalene which used methanol.
Furthermore, surprisingly I have found a process for preparing 2-methylnaphthalene in high selectivity which uses methanol. The temperature of this process is considerably higher than that taught by the prior art using methyl chloride.